"Male babbons use the size of the sexual swelling to determine motherly success," Domb said in Nature.

The scientists spent 13 months in Tanzania studying olive baboons. Among the baboons studied, females with the largest bottoms reached puberty the earliest and gave birth to more offspring. These children also had a better chance of surviving.

Domb also noted that "males fought more aggressively over females with larger swellings."

Domb siad the reason baboons publicize their fertility probably has to with the high rates of infanticide in the baboon population.

If females can mate with as many baboons as possible, there will be doubt about the paternity of the offspring. And baboon males are less likely to kill a baby they think is their own.

"Female baboons may be trying to mate with as many males as possible to confuse paternity and therefore avoid infanticidal action from nonfathers," Domb said.

Advertisting fertility is quite costly for female baboons. Swelling is so pronounced that a female can gain up to 14 percent of her body weight during ovulation. Swelling also hampers movement and makes her more vulnerable to infection.